Wire mesh insert for thermal adhesives

ABSTRACT

Curved surfaces of a preform of thermoplastic adhesive provide improved regulation of heating and exclusion of gas as surfaces to be bonded are heated and pressed against the thermoplastic adhesive preform. Volume and thickness of the bond are controlled by the inclusion of a wire mesh embedded in a preform or through which the thermoplastic adhesive is pressed during bonding. The wire mesh also increases heat transfer through the adhesive in a regulated and even manner over the area of the bond or any desired portion thereof. Particulate or filamentary materials can be added to the thermoplastic adhesive for adjustment of coefficient of thermal expansion or further increase of heat transfer through the adhesive or both. The preform is preferably fabricated by molding, preferably in combination with die-cutting of a preform of desired volume from a web of approximately the same thickness as the completed bond.

This is a divisional application of U.S. application Ser. No. 08/870,800filed on Jun. 6, 1997 U.S. Pat. No. 5,940,687.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to packaging of electroniccircuits, especially integrated circuits having high heat dissipationrequirements such as microprocessors and, more particularly, to theattachment of heat sinks to integrated circuit packages.

2. Description of the Prior Art

Heat dissipation is a major factor in the design of semiconductordevices such as analog and power transistors and especially in highperformance digital switching circuits formed at high integrationdensity. Ideally, in switching devices, heating is produced only duringthe switching transition interval and a small resistance in the "on"state. However, in high-performance circuits which provide very rapidswitching transitions, it is generally the practice to exploit rapidswitching by increasing the clock frequency so that the switchingtransition occupies a relatively constant portion of a clock periodregardless of switching speed. Therefore, heat dissipation generallyincreases with clock speed.

Further, to exploit this increased clock frequency and to obtain reducedsusceptibility to noise (as well as reduce manufacturing costs) there issubstantial incentive to fabricate high-performance switching elementsat the maximum possible integration density to minimize the length ofsignal propagation paths therebetween and to reduce noisesusceptibility. Therefore heat dissipation requirements also increaseproportionally with integration density.

Accordingly, it has recently become the practice to incorporateattachment of a heat sink or other heat removal structure (e.g. aliquid-cooled cold plate) into the design and manufacture of integratedcircuit packages since heat removal is critical to both performance andreliability of the integrated circuit. In this regard, incorporation ofthe heat sink with the fabrication of the package is justified by thecriticality of heat transfer from the package to the heat sink sinceuneven cooling may cause stresses within the chip or between the packageand the heat sink. Thermal cycling can permanently damage the circuitelements (e.g. by diffusion or oxide growth) or connections formed onthe chip (e.g. metal fatigue or migration) or degrade the attachment ofthe heat sink to the package which will then tend to increase thetemperature excursion during thermal cycling.

For attachment of heat sinks to integrated circuit packages, it has beenthe practice to use an adhesive which has a relatively good thermalconductivity. However, the thermal conductivity of such materials isstill very low compared to metals. For example, the thermal conductivityof a thermally conductive adhesive in current use is only about 1.73W/m-° C. whereas copper has a thermal conductivity of 395 W/m-° C.Additionally, the interfaces of the package to the adhesive and theadhesive to the heat sink further impede heat transfer. Therefore, itcan be understood that the adhesive connection of the heat sink iscritical to both the thermal and electrical performance of thecombination of chip, package and heat sink.

Specifically, the cross-section of the thermal path must be maximizedand should not be compromised by gas or air bubbles. Such bubblespresent a region of reduced thermal conductivity and two additionalinterfaces to impede heat flow. Further, thermal cycling causesexpansion of the gas or increase of pressure within the bubble which cancause progressive breakage of the adhesive bond.

Additionally, it is known that a certain volume of adhesive is necessaryto provide sufficient robustness of the bond to resist damage thereto byroutine handling before or after the package is placed in service andthe same applies to damage from gas or air bubbles, as well. On theother hand, since the thermally conductive adhesive has a significantthermal resistance, the length of the thermal path through the bondshould be no more that required by the volume of adhesive necessary to arobust bond. Therefore, the thickness of the adhesive bond is relativelycritical to the integrated circuit package.

It has therefore been the practice to bond heat sinks to integratedcircuit packages with a reworkable thermoplastic adhesive which isinitially in the form of a sheet of a thickness designed to provide theproper volume and thickness of the bond. In this sense, the sheet isessentially an adhesive preform and presents the problems of arequirement for heating the entire assembly to form the bond while heattransfer to the preform is low and irregular before the bond is formedand the possibility of capturing air or ambient gas at the surfaces ofthe sheet while the assembly is pressed together for heating andbonding. Throughput is low due to the thermal mass which must be heatedand cooled.

A dispensable adhesive is an alternative to an adhesive preform.Unfortunately, dispensable adhesives do not fully solve the problems ofa preform and present others. While air or gas will not generally betrapped by a dispensable adhesive that can flow when parts are pressedtogether, the thickness of the adhesive bond cannot be well-regulated.Further, good handling characteristics of dispensable adhesives such asease of dispensing, long storage and pot life and short cure timegenerally imply poor thermal performance and vice-versa. Poor thermalcharacteristics increase the criticality of the adhesive bond thickness.

Epoxies with suitable thermal conductivity, after mixing, must stayfrozen until use, require special dispensing equipment, have a shortworking life and require a long oven cure. Suitable cyanoacrylateadhesives also require special dispensing equipment, the addition of anactivator for curing and require only light handling, at most, forseveral hours after the bond is made. Either the long oven cure requiredby the epoxy or the period of restriction on handling of the devicecauses a restriction on the duration and throughput of the manufacturingprocess.

Accordingly, it can be seen that known alternatives for bonding heatsinks to circuit packages all present some unavoidable complexity in themanufacturing process and the possibility of compromising manufacturingyield or reliability.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide athermoplastic adhesive preform which avoids entrapment of air or gaseswhile providing precise control of adhesive volume and bond thickness.

It is another object of the invention to provide enhancement of heatconduction through an adhesive bond while improving accuracy andrepeatability of bond thickness.

It is a further object of the invention to provide for use of anadhesive which does not require dispensing, is reworkable without damageto an electronic circuit package and may be conveniently handled andstored prior to use for bonding heat sinks to electronic circuitpackages.

It is yet another object of the present invention to increase regularityof heat transfer to a thermoplastic adhesive preform during heating toform a bond.

In order to accomplish these and other objects of the invention, amethod of attaching a heat removal structure to an electronic circuitpackage is provided including the steps of forming a curved surface on apreform of thermoplastic adhesive, and heating and compressing theadhesive preform together with a wire mesh between said electroniccircuit package and said heat removal structure to form an adhesive bondhaving a desired thickness substantially equal to a thickness of saidwire mesh.

In accordance with another aspect of the invention, a method for makinga preform of thermoplastic adhesive having a curved surface and a wiremesh embedded therein is provided including the steps of placing adesired volume of thermoplastic adhesive and a wire mesh in a mold, andapplying heat and pressure to the volume of thermoplastic adhesive untilit conforms to a curved inner surface of said mold.

In accordance with a further aspect of the invention, an adhesivepreform of thermoplastic material is provided including a mesh ofthermally conductive wire, and a body of thermoplastic adhesive materialhaving at least one curved major surface for contacting a surface to bebonded over a progressively larger area as a bond having a thicknesssubstantially equal to a thickness of said wire mesh is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a cross-sectional view of a preferred form of a thermoplasticadhesive preform in accordance with the invention prior to bonding,

FIG. 1A is a plan view of the preform of FIG. 1,

FIG. 2 is a cross-sectional view of a completed bond between anelectronic circuit package and heat sink using the preform of FIG. 1,

FIG. 3 is a cross-sectional view of a variant form of an adhesivepreform before bonding having enhanced heat transfer capability inaccordance with the invention,

FIG. 3A is an alternative form of assembly of preforms in accordancewith the variant form of the invention shown in FIG. 3,

FIG. 4 is a cross-sectional view of a completed bond using the variantform of the preform of FIG. 3, and

FIG. 5 is a cross-sectional view of a preferred method of forming thethermoplastic preform of FIG. 1 or 3.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown, in cross-sectional view, an adhesive preform 30 in accordancewith the invention together with an electronic package 20 and a heatsink 40 which are to be bonded thereby. As alluded to above, when a bondis to be formed, the electronic package 20 and the heat sink 40 areassembled together with the thermoplastic preform 30 sandwichedtherebetween and the package and heat sink pressed toward each other(e.g. by support 10 and press 50) while the entire assembly is heated.Heating of the entire assembly can be reduced somewhat by preheating ofthe adhesive preform prior to assembly.

It can be readily appreciated that if the adhesive preform issubstantially flat, irregularities in the surface of the package 20,preform 30 and/or heat sink 40 will prevent intimate contact over theentire interfaces between these components prior to bonding. Thus whenthe surfaces are pressed together, ambient gases will be trappedresulting in compromise of both heat transfer and the integrity androbustness of the bond. At he same time, heat transfer from the packageand heat sink to the adhesive preform is irregular prior to bonding dueto lack of intimate contact to the preform. Further, it should beappreciated that during heating to form the bond while the assembly isunder compression, the adhesive will flow somewhat and can extrude fromthe edges of the bond. Therefore, a flat preform does not provideregulation of the amount of adhesive in the bond or the thickness of thebond, particularly where gas may be trapped between the package and orheat sink and the preform.

The same is especially true of dispensed adhesives where adhesive may bedistributed in lines or dots over either or both of the surfaces to bejoined. Such discontinuities in the pattern of adhesive, as applied, isparticularly subject to the trapping of ambient gases as the adhesive iscompressed and flows between the surfaces to be bonded. Trapped gas willthus cause extrusion of more than the intended amount of adhesive if theintended thickness of the bond is obtained, resulting in less than theintended amount of adhesive remaining in the bond, or, alternatively,the thickness of the bond will be increased by an amount correspondingto the volume of the trapped gas. Both of these effects or any degree ofcombination of the two compromises both heat transfer and bondintegrity.

To provide for avoidance of the trapping of gases in accordance with theinvention, preform 30 is formed with a slight degree of curvature in thetop and bottom surfaces thereof so that the preform is of a convex orpillow-like shape in cross-section. It is preferred that the amount ofcurvature be chosen such that the initial maximum thickness t₀ of thepreform be at least 1.5 times the thickness of the final designthickness, t, (FIG. 2) of the bond. Alternatively, the curvature shouldbe chosen in accordance with the dimensions of surface irregularities ofthe preform to avoid closing of any concave depression in the surface ofthe preform before gases can be excluded therefrom by deformation andflow of the preform such that the convex surface contacts aprogressively larger area of a surface to be bonded as a bond of desireddesign thickness is formed. It should also be appreciated that thecurvature provides an intimate contact between the central area of thepackage 20 and/or heat sink 40 with the central portion of the preform30 so that heating of the preform will be both rapid and progressive,aiding the exclusion of gases as the preform material is progressivelyallowed to flow. In a limiting case, the preform 30 could be in theshape of a sphere of appropriate volume to accommodate relativelyextreme surface roughness of the preform if such surface roughness wasnot otherwise avoidable.

It is also considered desirable in view of the viscous flow of theadhesive under compression and at a suitably elevated temperature thatthe perimeter of the preform 30 be slightly smaller than the dimensionsof surfaces to be bonded, as indicated at 25 of FIG. 1A, and slightlyconcave in plan view so that the final shape of the preform 30' willconform closely to that (25) of the areas to be bonded. Thus, since thevolume and shape of the adhesive can be controlled and trapping of gasescan be avoided, the onset of extrusion of adhesive 30' from between thesurfaces to be bonded can be detected used to regulate the finalthickness, t, of the bond, as shown in FIG. 2, by terminating heatingand compression and the cross-sectional area of the heat transfer pathwill be similarly well-regulated.

Referring now to FIG. 3, a variant form of the invention will now bediscussed. It was noted above that so-called heat conductive adhesivesare very poor conductors of heat as compared to copper and othermaterials such as other metals or alloys. Additionally, from the abovediscussion of shaped thermoplastic adhesive preforms in accordance withthe invention trapping of gases can be reliably avoided. Therefore,since the area of the bond is reliably maximized, some portion of thevolume of the bond may be utilized by a structure of greater heatconductivity to increase the average heat conductivity of the bondwithout significant compromise of the structural integrity or robustnessof the bond.

In a preferred form, the heat conductive structure to be included in thebond in accordance with the invention is in the form of a screen 60woven of a metal wire of high thermal conductivity such as copper or acopper alloy (or silver or gold if the additional heat conductivity ofsuch metals justifies their cost and accommodation of their greatermalleability can be accommodated in a particularly critical design).Preferably the diameter of the wire is chosen as one-half of the finaldesign thickness, t, of the bond so that the screen functions as astand-off during the bonding process and accurately establishes thethickness of the bond with high reliability and repeatability.

The pitch, p, of the wires in the woven screen is determined inaccordance with the reduction of volume and area of the bond which canbe tolerated without significant compromise of bond integrity orrobustness. In this regard, possible compromise of the robustness of thebond will be reduced as the coefficient of thermal expansion of theadhesive is matched (e.g. by choice of thermoplastic adhesive materialor the use of additional fill material 61 in a particulate or powderform) to that of the wire. However, the closeness of the match is not atall critical to the practice of the invention and should be regarded asa perfecting feature thereof, potentially allowing the screen materialto be of somewhat increased volume. As a further perfecting feature ofthe invention, particulate or filamentary form material 62 of highthermal conductivity can be added to the adhesive of the preform toincrease thermal conductivity beyond the increase provided by mesh 60.Particulate or filamentary material for either adjustment of coefficientof thermal expansion or increase of heat conductivity may be employedwhether or not screen 60 is provided. However, use with screen 60 ispreferred.

As shown in FIG. 3, the woven wire screen or mesh can be formed withinthe thermoplastic adhesive preform 30 by forming, molding or injectionmolding as will be discussed below. Alternatively, as shown in FIG. 3A,two thinner preforms 31, 32 can be used. In this latter case, while itis preferred that curvature be provided on both sides of each ofpreforms 31, 32 to reliably avoid trapping of gases, the curvature onthe side of the preforms facing the screen can be made substantiallyless or even flat while achieving that meritorious effect of theinvention. The curvature of the preforms on the side facing away fromscreen or mesh 60 together with the viscosity of the heatedthermoplastic adhesive and the viscous drag imposed by the mesh on thethermoplastic material being forced therethrough will provide aprogressive filling of the screen from the center outward to avoidsignificant gas entrapment between preforms 31 and 32 where they meetand merge within the screen or mesh 60.

As can be seen from the completed bond illustrated in FIG. 4, the wirescreen or mesh 60 provides a stand-off function between package 20 andheat sink 40 to accurately and repeatably establish the thickness, t, ofthe bond. It can also be seen that the outward flow of the adhesivematerial from the center of the preform toward the edge 33 of thecompleted bond occurs laterally through the mesh 60 which producessignificant viscous drag on the thermoplastic material. By the sametoken, however, the lateral extrusion of the thermoplastic materialthrough the mesh reliably provides for the progressive escape of gasesand the substantial filling of the interstices between wires in themesh, including the small tapered spaces 64 where the wires cross eachother in the mesh.

Referring now to FIG. 5, methods for forming the preforms of FIGS. 1, 3and 3A will be discussed. A preferred technique is to simply die cut andsimultaneously form the preforms 30 from a sheet or continuous web 80,which may have mesh sections 60 or particulate or filamentary additives61, 62 embedded therein, using a punch 70 and die 72 which have a shapedconcave interior. The web 80 can then advantageously be of approximatelythe same thickness as the intended thickness, t, of the completed bond.

The initial die cutting of the sheet or web 80 by punch blade 71 againstdie 72 establishes the amount of thermoplastic adhesive to form the bondand, as the punch 70 and die 72 are driven further together, thethermoplastic material, heated by heaters 74 and/or 75 is pinched at theedges and forced to flow (through the mesh 60, if included) toward thecenter of the preform or mold to form a pillow-like shape with curvedsurfaces. Gases are allowed to escape from the shaped interior of thepunch 70 and die 72 through vents 76. Thus, the preform is substantiallymolded to the desired shape and any residual stress in the preform willassist in causing adhesive flow in the preform which assists inachieving the desired shape of the completed bond (which will besubstantially the same as that when originally cut from sheet or web80). If a mesh 60 is included, this molding of the preform pushes thethermoplastic material laterally through the mesh in a directionopposite to that described above to improve thorough filling of themesh.

It should be understood that FIG. 5 can also be understood as beingillustrative of injection molding and other molding processes with theexception that in the latter case, a cutting edge 71 need not beprovided. The mold can also be charged with a preform of adhesivematerial of appropriate volume in an arbitrary shape (such as a sphereor a pellet of, for example, ellipsoid or cylindrical shape) for moldingto the preferred pillow shape with curved surfaces.

It should be noted from the foregoing, that the use of a thermoplasticmaterial allows the adhesive bond to be softened by the application ofheat so that a heat sink can be removed from an electronic circuitpackage without damage to the package. The bonding material can also beeasily removed from both the heat sink and the electronic package and,moreover, may be recovered and reformed by molding in the mannerdescribed above for reuse.

In view of the foregoing, it is seen that the invention provides forreliably and repeatably avoiding the entrapment of gases while forming areworkable bond of a heat removal structure such as a heat sink or coldplate to an electronic circuit package. The thermoplastic adhesivepreforms also provide for accuracy of bond volume and thickness, theavoidance of dispensing and ease of handling and storage of the adhesiveprior to use. Design dimensions of the bond to optimize bond integrityand robustness are accurately realized to optimize heat transfer whileallowing a portion of the volume and area of the bond to be allocated tostructures for increasing heat transfer capacity across the adhesivebond.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described my invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. An adhesive preform ofthermoplastic material includinga mesh of thermally conductive wire, anda body of thermoplastic adhesive material having at least one curvedmajor surface for contacting a surface to be bonded over a progressivelylarger area as a bond of desired final thickness is formed.
 2. Anadhesive preform as recited in claim 1, wherein a maximum initialthickness of said preform is at least 1.5 times said desired finalthickness of said bond.
 3. An adhesive preform as recited in claim 1,further includingparticulate material for adjusting a coefficient ofthermal expansion of said thermoplastic material.
 4. An adhesive preformas recited in claim 1, further includingparticulate or filamentarymaterial for increasing heat conductivity of said thermoplasticmaterial.
 5. An adhesive preform as recited in claim 3, furtherincludingparticulate or filamentary material for increasing heatconductivity of said thermoplastic material.
 6. An adhesive preform asrecited in claim 1, wherein said body of thermoplastic adhesive materialincludes a perimeter having concave shape edges.